1. Field of the Invention
The present invention relates to an optical communication system suitable for wavelength division multiplexing (WDM) communications utilizing a plurality of signal light components in a 1.58-xcexcm wavelength band (1565 nm to 1620 nm).
2. Related Background Art
WDM communications area type of optical communications enabling large-capacity communications by utilizing a plurality of signal light components having wavelengths different from each other. In WDM communications, a 1.55-xcexcm wavelength band (1530 nm to 1565 nm) has conventionally been utilized as a signal light wavelength band in the WDM communications since silica type optical fibers, which are typically used as a transmission line, have a low transmission loss in the 1.55-xcexcm wavelength band, and since Er-doped optical fiber amplifiers (EDFA: Er-Doped Fiber Amplifier) have a large gain in the 1.55-xcexcm wavelength band.
The optical transmission line employed in WDM communications in the 1.55-xcexcm wavelength band can utilize not only the above-mentioned silica type optical fibers, but also single-mode optical fibers having a zero-dispersion wavelength in a 1.3-xcexcm wavelength band (1260 nm to 1350 nm), dispersion-shifted optical fibers having a zero-dispersion wavelength in the 1.55-xcexcm wavelength band, or hybrid transmission lines in which these optical fibers are mixed. Since the above-mentioned single-mode optical fibers have a large positive dispersion in the 1.55- xcexcm wavelength band, dispersion-compensating optical fibers having a large negative dispersion in the 1.55-xcexcm wavelength band are utilized as a dispersion compensator for compensating for the dispersion of the single-mode optical fibers in the 1.55-xcexcm wavelength band.
The inventors have studied conventional optical communication systems in detail and, as a result, have found problems as follows.
Namely, since dispersion-shifted optical fibers exhibit a very small absolute value of dispersion (substantially zero) in the 1.55-xcexcm wavelength band and have a small effective area in general, signal light waveforms are likely to deteriorate due to nonlinear optical phenomena, such as four-wave mixing in particular, in the case of WDM communications in the 1.55-xcexcm wavelength band. Since such deterioration of signal light waveforms caused by nonlinear optical phenomena cannot be restored, it is necessary that the occurrence of such nonlinear optical phenomena be suppressed as much as possible. For suppressing the occurrence of nonlinear optical phenomena, the power of signal light maybe lowered. In this case, however, repeater intervals have to be shortened in long-distance optical communications, whereby the cost rises. Therefore, as another potent method, optical communications may be carried out in a wavelength band, different from the 1.55-xcexcm wavelength band, where dispersion occurs to a certain extent.
On the other hand, further larger capacity is demanded in the field of optical communications. From this viewpoint of achieving a larger capacity, the research and development aimed at widening the bandwidth that can be amplified by optical fiber amplifiers or utilizing wavelength bands other than the 1.55-xcexcm wavelength band has been under way. As optical fiber amplifiers which can amplify signal light in wavelength bands other than the 1.55-xcexcm wavelength band, those capable of amplifying signal light in the 1.58-xcexcm wavelength band have been realized, for example.
From the background as in the foregoing, WDM communications in the 1.58-xcexcm wavelength band in place of or in addition to the 1.55-xcexcm wavelength band have been considered for practical use. In this case, the transmission loss of silica type optical fibers is relatively low even in the 1.58-xcexcm wavelength band, whereby there are no inconveniences in particular.
Configurations of optical communication systems which transmit signal light in the 1.58-xcexcm wavelength band are described, for example, in a literaturexe2x80x94A. K. Srivastava, et al., ECOC""98, postdeadline paper, pp. 73-75 (1998)xe2x80x94, a literaturexe2x80x94Yano, et al., ECOC""98, pp. 261-262 (1998)xe2x80x94, a literaturexe2x80x94T. Sakamoto, et al., OAA""98, TuB3, pp. 88-91 (1998)xe2x80x94, and a literaturexe2x80x94M. Jinno, et al., IEEE Photon. Technol. Lett., Vol. 10, No. 3, pp. 454-456 (1998)xe2x80x94, for example. In each of the optical communication systems described in these literatures, the optical transmission line is constituted by a dispersion-shifted optical fiber alone. Since the dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-xcexcm wavelength band has a dispersion with an absolute value of about 2 to 3 ps/nm/km in the 1.58 -xcexcm wavelength band, four-wave mixing is relatively hard to occur therein.
On the other hand, dispersion-shifted optical fibers have a transmission loss slightly higher than that of optical fibers yielding a low transmission loss, such as single-mode optical fibers having a zero-dispersion wavelength near a wavelength of 1.3 xcexcm. Therefore, if such a low-loss optical fiber is employed in a part behind a dispersion-shifted optical fiber, then total loss can be lowered. However, the low-loss optical fibers have a dispersion with a large absolute value in the 1.58-xcexcm wavelength band in general. In this case, if a system is constructed carelessly, then it may not function as an optical communication system.
For solving the problems mentioned above, it is an object of the present invention to provide an optical communication system comprising a structure for effectively restraining signal light waveforms from deteriorating due to the occurrence of nonlinear optical phenomena, such as cross-phase modulation in particular, in WDM communications in the 1.58-xcexcm wavelength band even when it is an optical communication system including a dispersion-shifted optical fiber having a zero-dispersion wavelength in the 1.55-xcexcm wavelength band.
The optical communication system according to the present invention comprises at least one hybrid transmission unit. As a first configuration, this hybrid transmission unit has a dispersion-shifted optical fiber and a first high-dispersion optical fiber, whereas the dispersion-shifted optical fiber is disposed upstream the first high-dispersion optical fiber such that a WDM signal successively propagates through the dispersion-shifted optical fiber and the first high-dispersion optical fiber. The dispersion-shifted optical fiber is an optical fiber with a length LDSF having a zero-dispersion wavelength in the 1.55-xcexcm wavelength band (1530 nm to 1565 nm) and exhibiting, at a wavelength of 1.58 xcexcm, a dispersion DDSF with an absolute value of 0.5 ps/nm/km or more. The first high-dispersion optical fiber is an optical fiber with a length L1 exhibiting, at a wavelength of 1.58 xcexcm, a dispersion D1 having an absolute value greater than that of the dispersion DDSF of the dispersion-shifted optical fiber. Here, the dispersion-shifted optical fiber and the first high-dispersion optical fiber may be disposed such that a repeater including a coupler and an optical amplifier, for example, is interposed therebetween.
In particular, with respect to at least signal light having the shortest wavelength in signal light having a bit rate B included in a signal light wavelength band in which wavelength ranges from 1.565 xcexcm to 1.610 xcexcm, the hybrid transmission unit of the first configuration comprising the dispersion-shifted optical fiber and first high-dispersion optical fiber satisfies the following condition:
xcex94xcfx86XPMxc2x7DTxe2x89xa618000 (unit:(ps/nm)xc2x7(Gb/s)2)
DT=(DDSFxc2x7LDSF+D1xc2x7L1)xc2x7B2
where xcex94xcfx86XPM is the total phase shift amount of cross-phase modulation in the signal light having the shortest wavelength under the influence of signal light having the other wavelengths, and DT is the total dispersion in the hybrid transmission unit.
On the other hand, the optical communication system according to the present invention may comprise a structure for compensating for the dispersion occurring in the system. In this case, as a second configuration, the hybrid transmission unit comprises a second high-dispersion optical fiber in addition to the dispersion-shifted optical fiber and first high-dispersion optical fiber. This second high-dispersion optical fiber is an optical fiber with a length L2 exhibiting, with respect to light having a wavelength of 1.58 xcexcm, a dispersion D2 having an absolute value greater than that of the dispersion DDSF of the dispersion-shifted optical fiber and a polarity different from that of the dispersion D1 of the first high-dispersion optical fiber; and is disposed in one of a transmission line (upstream the dispersion-shifted optical fiber) through which light to enter the dispersion-shifted optical fiber propagates, a transmission line between the dispersion-shifted optical fiber and the first high-dispersion optical fiber, and a transmission line (downstream the first high-dispersion optical fiber) through which light emitted from the first high-dispersion optical fiber is to propagate.
With respect to at least signal light having the shortest wavelength in signal light having a bit rate B included in a signal light wavelength band in which wavelength ranges from 1.565 xcexcm to 1.610 xcexcm, the hybrid transmission unit of the second configuration comprising the dispersion-shifted optical fiber, first high-dispersion optical fiber, and second high-dispersion optical fiber as such satisfies the following condition:
xe2x80x83xcex94xcfx86XPMxc2x7DT less than 18000 (unit:(ps/nm)xc2x7(Gb/s)2)
DT=(DDSFxc2x7LDSF+D1xc2x7L1+D2xc2x7L2)xc2x7B2
where xcex94xcfx86XPM is the total phase shift amount of cross-phase modulation in the signal light having the shortest wavelength under the influence of signal light having the other wavelengths, and DT is the total dispersion in the hybrid transmission unit.
In each of the above-mentioned configurations, if N ( greater than 2) channels of signal light propagate through the optical communication system, then the total phase shift amount xcex94xcfx86XPM(i) resulting from the cross-phase modulation in signal light having a wavelength xcexi (i=1, 2, . . . , N) incident on the dispersion-shifted optical fiber under the influence of signal light having a wavelength xcexj (j=1, 2, . . . , N; jxe2x89xa0i) is given by the following expression:       Δ    ⁢          xe2x80x83        ⁢          φ      XPM        ⁢          xe2x80x83        ⁢          (      i      )        =      2    ·    γ    ·          L      eff        ·                  ∑                  (                                    j              =              1                        ,                          j              ≠              i                                )                N            ⁢              xe2x80x83            ⁢              (                  P          ⁢                      xe2x80x83                    ⁢                                    (              j              )                        ·                                                            η                  XPM                                ⁢                                  xe2x80x83                                ⁢                                  (                                      i                    ,                    j                                    )                                                                    )            
where xcex3 is the nonlinear coefficient in the dispersion-shifted optical fiber, Leff is the effective length of the dispersion-shifted optical fiber, P(j) is the peak power of each of (Nxe2x88x921) channels of signal light excluding the wavelength xcexi, and xcex7XPM(i,j) is the efficiency of occurrence of cross-phase modulation between the wavelengths xcexi and xcexj of signal light. If two channels of signal light having respective wavelengths xcex1 and xcex2 propagate through the hybrid transmission unit, on the other hand, then the total phase shift amount xcex94xcfx86XPM(1) resulting from the cross-phase modulation in the signal light having the wavelength xcex1 incident on the dispersion-shifted optical fiber under the influence of the signal light having the wavelength xcex2 is given by the following expression:
xcex94xcfx86XPM(1)=2xc2x7xcex3xc2x7Leffxc2x7(P(2)xc2x7{square root over (xcex7XPM(1,2))})
Further, the effective length Leff of the dispersion-shifted optical fiber and the efficiency of occurrence of cross-phase modulation xcex7XPM(i,j) between the wavelengths xcexi and xcexj of signal light are given by the following expressions:
Leff=(1xe2x88x92exp(xe2x88x92xcex1xc2x7LDSF))/xcex1
            η      XPM        ⁢          xe2x80x83        ⁢          (              i        ,        j            )        =                    α        2                                                        Ω              m              2                        ·            d                    ⁢                      xe2x80x83                    ⁢                                    (                              i                ,                j                            )                        2                          +                  α          2                      ·          [              1        +                                            4              ·                              sin                2                                      ⁢                          xe2x80x83                        ⁢                                          (                                                                            Ω                      m                                        ·                    d                                    ⁢                                      xe2x80x83                                    ⁢                                                            (                                              i                        ,                        j                                            )                                        ·                                                                  L                        DSF                                            2                                                                      )                            ·                              ⅇ                                                      -                    α                                    ⁢                                      xe2x80x83                                    ⁢                                      L                    DSF                                                                                                          (                              1                -                                  ⅇ                                                            -                      α                                        ⁢                                          xe2x80x83                                        ⁢                                          L                      DSF                                                                                  )                        2                              ]      
where xcex1 is the transmission loss in the dispersion-shifted optical fiber, xcexa9m is the modulation frequency, and d(i,j) is the delay time difference per unit distance between the wavelengths xcexi and xcexj of signal light.
In the optical communication system according to the present invention, the hybrid transmission units of the first and second configurations satisfy a condition under which the product xcfx86XPMxc2x7DT becomes 18000 (ps/nm)xc2x7(Gb/s)2 or less with respect to the shortest wavelength of signal light included in the signal light wavelength band. As a consequence, signal light waveforms are restrained from deteriorating due to interactions between the occurrence of nonlinear optical phenomena, such as cross-phase modulation in particular, and dispersion. In a system having secured an appropriate SN ratio, its power penalty becomes about 1.0 dB or less, and a bit error rate of 10xe2x88x929 or less can be achieved. If the product xcfx86XPMxc2x7DT becomes 13000 (ps/nm)xc2x7(Gb/s)2 or less with respect to the shortest wavelength of signal light included in the signal light wavelength band, then the above-mentioned power penalty becomes substantially 0.2 dB or less, and the above-mentioned bit error rate can attain 10xe2x88x9215 or less. From the viewpoint of suppressing the above-mentioned power penalty and bit error rate, the product xcfx86XPMxc2x7DT is most preferably 13000 (ps/nm)xc2x7(Gb/s)2 or less with respect to all of the signal light included in the signal wavelength band.
In particular, the hybrid transmission unit of the second configuration including the second high-dispersion optical fiber having the dispersion D2 with a polarity opposite to that of the dispersion D1 of the first high-dispersion optical fiber can lower the parameter DT (total dispersion amount in the hybrid transmission unit) and, consequently, the product xcfx86XPMxc2x7DT, thereby being able to remarkably reduce both power penalty and bit error rate.
Preferably, the dispersion-shifted optical fiber included in each of the hybrid transmission units of the first and second configurations has a positive dispersion at a wavelength of 1.58 xcexcm. In this case, waveforms of signal light enhance their eye under the pulse compression, which is desirable in terms of transmission. However, it would yield a higher peak power, whereby signal light waveforms are more likely to occur due to nonlinear optical phenomena. If the hybrid transmission units are designed such that the product xcfx86XPMxc2x7DT falls within the above-mentioned ranges, however, then favorable transmission characteristics can be secured.
Preferably, in the hybrid transmission system of the second configuration, one of the first and second high-dispersion optical fibers is a single-mode optical fiber having a zero-dispersion wavelength in the 1.3-xcexcm wavelength band, whereas the other is a dispersion-compensating optical fiber for compensating for the dispersion of the single-mode optical fiber in the signal light wavelength band. If an optical amplifier for amplifying signal light included in the signal light wavelength band is disposed upstream the dispersion-shifted optical fiber in the hybrid transmission unit of the second configuration, then the dispersion-compensating optical fiber is preferably disposed upstream the optical amplifier. This is because of the fact that, if the dispersion-compensating optical fiber is disposed upstream the optical amplifier, then the power of signal light propagating through the dispersion-compensating optical fiber is lowered, whereby nonlinear optical phenomena can effectively be restrained from occurring in the dispersion-compensating optical fiber having a high nonlinear characteristic in general. As a consequence, signal light waveforms can effectively be kept from deteriorating. Also, the dispersion-compensating optical fiber having a large negative dispersion widens the pulse waveform of signal light, and effectively lowers its peak power. Therefore, the signal light having a relatively lowpeak power would propagate through the dispersion-shifted optical fiber after being amplified by the optical amplifier, whereby the occurrence of nonlinear optical phenomena in the dispersion-shifted optical fiber can be suppressed (signal light waveforms can be kept from deteriorating) according to this configuration as well.
Preferably, in the hybrid transmission unit of the first configuration, each of the dispersion-shifted optical fiber and first high-dispersion optical fiber has a polarization mode dispersion of 2 psxc2x7kmxe2x88x92xc2xd or less in the signal light wavelength band. In the hybrid transmission unit of the second configuration, it is preferred that, in the signal light wavelength band, each of the dispersion-shifted optical fiber and first high-dispersion optical fiber have a polarization mode dispersion of 2 psxc2x7kmxe2x88x92xc2xd or less, and the second high-dispersion optical fiber have a polarization mode dispersion of 1 psxc2x7kmxe2x88x92xc2xd or less. Preferably, in each of the hybrid transmission units of the first and second configurations, the hybrid transmission unit has a total accumulated polarization dispersion of 1/(4B) or less in the signal light wavelength band. Each of these cases can more effectively restrain signal light waveforms from deteriorating due to cross-phase modulation and dispersion which depend on the polarization state of signal light.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.